Hydromechanical transmissions have been utilized to obtain braking torque as desired. However, systems used up to now, such as, for example, that disclosed in commonly assigned U.S. Pat. No. 3,261,232, do not provide any capability for regulating braking except by connecting different mechanical gears. Conventional hydromechanical transmissions have heretofore utilized a separate "retarder". Although such arrangements provide regulation or modulation of the braking torque, the retarder, which comprises a variably filled hydrodynamic coupling, provides considerable resistance against rotation when not in use and only filled with air. While there are techniques for reducing this loss of the air filled retarder, the arrangement for accomplishing this requires considerable space and is heavy and expensive.
As stated above, the prior art system referred to above would not regulate braking itself. At best, one could obtain different degrees of braking capacity by different settings of the torque converter and by utilizing different mechanical gear connections. However, the method of the present invention provides for regulating the braking force between wide limits and with less shock when changing between mechanical gear ratios, and does not depend on any additional mechanical structures except that concerning the automatic control for providing a predetermined braking setting.